In general terms the thermoforming process relates to the process of forming a shaped article by stretching a heated thermoplastic sheet material such as, for example, polyethylene or polypropylene into a mold by the use of differential air pressure. In one of its forms thermoforming relates to the formation of a relatively shallow article by the use of vacuum and/or air under pressure to stretch and thus force a preheated thermoplastic sheet material into a forming cavity of a mold. When the shaped article is required to have a greater depth than the beforementioned relatively shallow article, a plug or tool has been utilized to assist in the stretching of the heated thermoplastic sheet material down into the forming cavity of a mold. Thereafter, formation of the article is completed by application of a vacuum and/or air under pressure to force close conformance of the thermoplastic sheet material with the surface of the forming cavity of the mold. The actual size and shape of the plug used is dependent on the size and shape of the article to be manufactured. One objective in using such a plug is to uniformly stretch the thermoplastic sheet material so that consistent thinning of the sheet material occurs and the final shaped article, i.e. product, has a wall cross-section or thickness which is as consistent and even as possible.
In several known methods of forming shaped containers by the use of a thermoforming plug, the geometric design of the plug and the timing of its downward stroke relative to the introduction of forming air and/or application of vacuum have been used in attempting to control the degree of container wall cross-section uniformity and, accordingly, the end use and physical or structural performance of the manufactured container or other article. Plugs utilized in the past have generally been of a solid construction and manufactured from a single material. Examples of plug materials which have been utilized are wood, felt, aluminum, nylon, polyacetyl (Delrin), and filled epoxy. Unfortunately, the presently known designs for thermoforming plugs have been limited in their degree of control over the consistency of the stretching of the heated thermoplastic sheet material and thus the consistency of the thickness of the cross-section of the wall of a product. Thus, their ability to improve the structural performance of a container or other article without a significant weight and therefore cost penalty has also been limited. Simply stated, existing plug designs have been limited in their ability to selectively strengthen, via selectively increased wall thickness, a critical preformance area of a container or other article without increasing the thickness of other areas of the article. Thus, in the past, many containers have been much thicker and heavier than required in non-critical areas and have utilized an excessively large amount of raw material since such action was necessary to effect a satisfactory thickness, strength, rigidity or other structural performance characteristic in a critical performance area. By way of example only, the weight distribution of one known plastic container formed by utilizing an existing plug design is lid/stacking ledge 20%, wall section 58% and base 22%. Although the figure of 22% for the base is too high and a figure of 8 to 10% would be adequate for satisfactory performance of the container, such a distribution is, to a large extent, dictated by the use of existing plug designs.
While the use of plugs or tools of the sort discussed above has, to some extent, resulted in a more consistent stretching and thus distribution of the thermoplastic sheet material throughout the whole shaped article, wall thickness and distribution inconsistencies have, for the reasons discussed above, continued. This lack of control over both the distribution of the thermoplastic material to certain key areas of the article and the consistency of the thermoplastic material has become more important in recent years since the cost of thermoplastic raw materials has been increasing faster than other manufacturing costs and the raw material cost now represent the major cost of a light weight, usually one-trip, disposable plastic container. Accordingly, a continuing quest directed to discovering improved methods and apparatus for manufacturing, on a more cost efficient basis, containers of this sort has been underway. The quest has led those skilled in the art down many avenues. For example, one approach is illustrated in U.S. Pat. No. 3,901,640 to Tigner et al. This approach provides an expandable forming plug enabling variable wall thicknesses in the hollow article by control of the expansion of the plug during the forming of the article. Another approach is illustrated in U.S. Pat. No. 2,990,581 to Rowe, Jr. This approach involves the simultaneous mechanical forming of the article by a plug coupled with differential gas pressure drawing by the application of differential gas pressure through a passage in the plug. Yet another approach has combined the use of a novel plug configuration with the application of differential gas pressure through the plug. This latter approach is illustrated in U.S. Pat. No. 4,039,271 to Hudson et al.